Fix support for vector<bool> serialization with libc++ #93

The vector<bool> serialization support added in https://github.com/chaincodelabs/libmultiprocess/pull/86 commit 1c6fb04dddfc0de35eb5952649fda83a030ff288 was only tested with libstdc++ not libc++ and caused a test compilation failure when compiling on macos, reported by edilmedeiros in https://github.com/chaincodelabs/libmultiprocess/issues/92

Fix the issue that was causing the build to fail: trying to use using vector's reference type instead of const_reference type when serializing the vector.

Fixes #92

I confirm that the changes in 8da797c now make the tests compile and pass in my box. I compiled and checked them by getting dependencies from both Brew and Macports.